The field of this invention is a system and method for determining the attenuation of electromagnetic waves by an electromagnetic shield, which may include any material or path designed to cause, or resulting in, any partial or complete loss of electromagnetic signal strength.
Shielded enclosures or shelters are utilized in various industries to shield electromagnetic waves from propagating through the shield of a shelter or enclosure. For example, an electromagnetic shield can be used for security purposes where it is necessary to prevent electromagnetic emanations from leaving the enclosure or shelter or to prevent electromagnetic emanations from entering the enclosure or shelter. In the case of a shielded building, for example, it may be important to shield every wall in the building. Shielded enclosures may also be used with medical diagnostic equipment such as MRI (magnetic resonance imaging) equipment where it may be important to prevent outside electromagnetic emanations from affecting the equipment's ability to perform accurately. They may also be used for rooms that are used to test electronic equipment where it is important to prevent electromagnetic emanations either from entering the room or from leaving the room.
It is often important to test the ability of the shielded enclosure to perform its shielding function. The effectiveness of the shielded enclosure is measured by introducing electromagnetic waves of known amplitude and frequency on one side of the shielded enclosure and receiving the electromagnetic waves on the other side of the shielded enclosure. The difference or other relationship between the two amplitudes is a measure of the attenuation of the shield. Some methods and equipment that may be used to perform such a test is disclosed in MIL-STD-285, Jun. 25, 1956, in NSA 65-6, and in NSA 65-5. The higher the attenuation the greater the effectiveness of the shield. Thus, attenuation is a measure of shielding effectiveness.
Various shielding materials and techniques perform differently depending on the type of electromagnetic field and the frequency of the waves being generated. Because each frequency range has its own particular characteristics, an electromagnetic shield will react differently to different waves exhibiting different frequencies. Exemplary materials can include exotic metals, other metals, air, fluids and other shielding materials.
In order to perform electromagnetic testing on shielded shelters or enclosures, two subsystems are used, one subsystem to generate the waves on one side of the shield or enclosure and another subsystem on the other side of the shield or enclosure to receive the waves. The waves used for testing may be generated in various frequency ranges. One such range used for testing is a microwave frequency range that may vary from about 10 GHz to about 10.3 GHz. A difficulty with using a frequency in this range is that corresponding signals do not propagate through wire without a high level of loss. Standard coaxial cable (such as RG58 or other lower cost cable) has a high level of loss through the cable. Utilizing low loss cable improves the problem of high loss but does not eliminate it and poses additional problems. Low loss cable is not practical due to its fragile construction and stiffness and is damaged very easily. In both cases (low cost and low loss cable) the cable lengths must be kept short to minimize loss of signal amplitude which results in decreased testing capability in the form of loss of dynamic range. Dynamic range is defined as the highest level to the lowest level of signal amplitude that can be measured.
The generation side of the system can sometimes be kept close to the transmitting antenna thereby minimizing the loss of signal amplitude between a source of waves and an antenna that transmits the waves. But on the receiving side of the system, the receiving antenna must be moved to various positions about the shelter or enclosure and preferably not be kept close to an analyzer without significant difficulty. Moving the receiving antenna can be done; but this necessitates moving the receiver electronics along with the receiving antenna which is cumbersome and takes time during the testing.